Thermostats are easy to replace. Essentially 2 bolts, pull out the old one, scrape the intake and outlet, drop the new one in, spread a little silicone, and re bolt it. Allow it to sit for a while to allow the silicone to dry, and you're good to go!

If you're just going to be replacing it, any autoparts store will be able to pull up the exact replacement off the computer. If you're modifying it, keep asking around for others input.

Year, make, model would help some since the ease of replacing does have a factor.
On the new vortec motors 1996 to present there is an o-ring instead of a gasket so it takes less than 5 min to do the entire project. Older than 95 you have to replace the gasket and scrape etc..
all of the 350 V-8s from 1988 up took a 195degree t-stat, as well as the 4.3L 4.6L and 5.3L as well as the 6.0L